Question

Rishi went to a palmist to show his palm. The palmist used a special lens
for this purpose.
(1) State the nature of the lens and reason for its use.
(ii) Where should the palmist place/hold the lens so as to have a real and
magnified image of an object ?
(iii) If the focal length of this lens is 10 cm and the lens is held at a
distance of 5 cm from the palm, use lens formula to find the position
and size of the image.​

Answer 1

Answer:

the nature of the lens will be Convection because it forms virtual erect image

Answer 2

(i) The nature of the lens is converging lens, i.e., it is a convex lens. It is used to magnify the image of the palm so that it can be read easily.

(ii) The palmist should place/hold the lens between focus (f) and centre of curvature (or 2f) so as to have a real and  magnified image of an object.

(iii) Given:

f = + 10 cm

u = -5 cm

To Find:

Image distance v

Calculation:

Using lens formula, we have

1/f = 1/v - 1/u

⇒ 1/10 = 1/v - 1/(-5)

⇒ 1/v = 1/10 - 1/5

⇒ 1/v = (1-2)/10

⇒ 1/v = -1/10

⇒ v = -10 cm

Now, magnification, m = -v/u

⇒ m = -(-10)/(-5)

⇒ m = -2  (-ve sign shows virtual image)

- So the image is formed at 10 cm to the left of the lens and the size of the image will be double the size of palm.